1. Field of the Invention
The present invention relates to a method for preparing biodegradable polyester using compressed gas as a reaction solvent, and biodegradable polyester prepared thereby.
2. Description of the Background Art
A degradable polymer material is much favored in various fields such as the medical science, the science of agriculture, and environment, due to its specific degradable characteristics. Especially, the degradable polymer material has sharply increasing values in the field of the environment and the medical science.
The biodegradable polymer is roughly divided into a natural biodegradable polymer and a synthetic biodegradable polymer. Out of them, the natural biodegradable polymer has a high affinity to the environment, and a high physical property and adaptability to a living body, due to being made of natural materials, and for these reasons it is recognized as a preferable material. But, the natural biodegradable polymer has such problems that it is high-priced and cannot be arbitrarily controlled due to the characteristics of the natural material.
Meanwhile, recently, the synthetic biodegradable polymer is highly commercially valued in the aspect that it can be artificially controlled unlike the natural biodegradable polymer, to complement the above problems of the natural biodegradable polymer. Among the synthetic biodegradable polymer materials, polyglycolide (PGA), polylactide (PLA) or the like are comparatively excellent in its property, and variously used in the environment and medical fields, due to its high affinity and innoxiousness to the environment and the living body. Especially, the synthetic biodegradable polymer material is usable in the environment-related fields such as a disposable wrap film, an agricultural and industrial film, and food packing container. Further, the synthetic biodegradable polymer material has been developed and used in the medical field such as a drug delivery system (DDS), a pins, screw and suture for fixing bone and tissue.
In order to prepare the synthetic biodegradable polymer with high molecular weight, in general, a solid polymerization process is conducted using only monomer and catalyst, without solvent, wherein the monomer is polymerized at below a melting temperature of polymer. However, such solid polymerization process inevitably results in an ununiformity of physical properties, an adulteration of low molecular weight materials and a lowering of workability. In particular, in order to apply a polymer prepared in the large-scaled solid state to a shaping process, the polymer should be crushed to a small size, which makes a preparation process complicate in a mass-production of a commercial scale, and increases a production cost. In addition, in the use for the DDS, it is necessary that the form and the size of particles should be freely controlled in order to arbitrarily control a speed of a medicine-release.
In order to prepare polylactide/polyglycolide copolymer usable as a biodegradable and biocompatible material, researches have been conducted for a ring opening polymerization method by using alkyl lactate monomer, stabilizer, metal compound catalyst, and supercritical carbon dioxide as a dispersion medium (Macromol. Rapid Commun, 20, 511-514, 1999, J. Polym. Sci. Part A: Polym. Chem. 39, 562-570, 2001). Because polylactide/polyglycolide copolymer prepared by using the above method is insoluble to the supercritical carbon dioxide, a fluoropolymer is used as a stabilizer. However, in this method, when a reaction time is 48-72 hours, the molecular weight of the copolymer is 28,000-30,000 g/mol, indicating that the molecular weight is very low for the long reaction time.
Thus, the existing biodegradable polyester polymerized with the supercritical carbon dioxide as a dispersion medium is insoluble to the supercritical carbon dioxide used, to involve a long reaction time and a low molecular weight. Further, a stabilizer is needed in the preparation process thereof, to involve an additional process for removing the stabilizer after the reaction is finished.
Meanwhile, carbon dioxide as the supercritical fluid is frequently used for the reason of its low critical temperature and critical pressure, a low cost, incombustibility and innoxiousness. However, the supercritical carbon dioxide is not able to dissolve polymers except for fluoride-containing polymers and silicon-containing (e.g., siloxane) polymers.
Hydrocarbons and hydrochlorofluorocarbons (HCFCs) are known to well dissolve various polymers with high molecular weight when being used as a solvent. Meilchen et al. observed that poly(methylacrylate) and poly(ethylene-co-methacrylate) was dissolved in hydrochlorofluoromethane (HCFC-22). Haschets and Shine observed a phase change of poly(methylmethacrylate) and polycaprolactone in the presence of HCFC-22. The phenomenon that above polymer is dissolved in HCFC-22 occurs presumably by a special interaction between hydrogen atom in HCFC-22 and an ester group in polymer.
In order to resolve the aforementioned problems of the conventional methods for preparing biodegradable polyester in the solid process or using the supercritical CO2, the present invention provides the polymerization system consisting of an hydrofluorocarbons (HFCs) and/or HCFCs and/or dimethylethers, and the like, which are in a supercritical state and can be used for a solution-polymerization.
According to the present invention, compressed gas capable of dissolving polymer is used as a reaction solvent, to prepare polymer in a particle form with a high molecular weight within a short reaction time. In addition, differently from the conventional polymerization method using the supercritical carbon dioxide as the dispersion medium, the present invention does not need a stabilizer, to omit a process for removing the stabilizer, and thereby, the preparation process can be more simplified.
After the polymerization is completed as above, in a follow-up process, a product in a reactor is sprayed into a precipitator, and a non-solvent which dissolves a reaction solvent without dissolving the polymer to form a single phase is introduced. Thereby, a biodegradable homopolymerized polyester and a biodegradable copolymerized polyester with high molecular weight can be obtained in a fine powder form with a particle size of 0.01 to 1000 μm. That is, according to the present invention, a biodegradable polyester with a high molecular weight can be synthesized by a single consecutive process.